Successful drilling, completion and production of an earthen wellbore requires that information be gathered about the downhole formation from which hydrocarbons are produced. Measurement systems are lowered into a drilled wellbore to determine wellbore parameters and operating conditions. A portion of the measurement system includes a sensor package for detecting the wellbore parameters and conditions, such as formation properties, tool and borehole direction, drilling fluid properties, dynamic drilling conditions, and others. The sensor package may be lowered on a tool body after the drill string is tripped out of the borehole, such as with a typical wireline operation. Alternatively, the sensors may be housed in a drill collar and adapted for taking measurements while drilling, as in certain applications known as measurement-while-drilling (MWD) or logging-while-drilling (LWD). In addition to the sensor portion, a sensor tool may also include a processor and associated storage medium for retaining the sensed information. With respect to a MWD/LWD tool, a telemetry system is often used to transmit the sensed information uphole. The telemetry system may include a mud pulser, an acoustic telemetry option, or an electromagnetic transmission system.
The sensors and associated electronic and mechanical components are packaged within the tool body. For example, the sensors and detectors may be hardwired within the tool body and accessible via removable hatches. In another arrangement, the sensors are mounted upon a chassis and retained within an outer housing. However, such sensor packages are restricted by limited accessibility, wherein the sensor package components are accessed by disassembly of tool body parts or additional features such as access ports. They are not easily removed and/or replaced.
Specifically with respect to MWD/LWD tools, there are high capital and operating costs, and the tools must be adaptable to varying drill string sizes. Furthermore, the drilling environment is very dynamic with fluctuating pressures and temperatures, making precision measurements by the sensors difficult. Thus, the sensor package must provide robust isolation from the drilling environment, including a good pressure seal between the sensors and the environment exterior of the drill collar.
Sensors have been placed in insert-type packages wherein a housing receives a sensor case and a cover or sleeve is disposed over the housing to retain the sensor cases. These sensor cases are termed “inserts” because they are internal to the tool (within the cover or sleeve) and, if sealed, are dependent on the cover or sleeve or other external pressure case for sealing from the environment exterior of the tool. An insert is not accessible from an exterior of the tool. Some tools provide a pocket on the outside of the tool body and a sensor case that is placed in the pocket. Such a sensor case is accessible from an exterior of the tool, thus it is termed an “outsert.” The outsert may be sealed by an external pressure case, such as a hatch that fits into the pocket opening and seals the pocket. However, such external pressure cases are unreliable.
The high capital and operating costs of measurement tools, particularly the MWD/LWD type, require that sensor packages provide easy removability and replaceability of the sensors, flexibility to be used in measurement tools of various sizes, and robust sealing from the downhole environment. Despite the aforementioned advances, the current sensor packages are limited in such a way that this combination of parameters cannot be met.